Occurrence of endangered tri-spine horseshoe crab in Etajima and Takehara Cities of Hiroshima Prefecture, western Japan

The tri-spine horseshoe crab Tachypleus tridentatus is considered at risk and has been granted protection in Japan at least since the 1990s. The breeding areas in Okayama, Ehime and Saga prefectures, Japan, are protected by national and local laws, while in Hiroshima Prefecture the species has not been officially conserved by local administrations. However, its breeding has long been noticed at two sites there: Etajima and Takehara cities. We compiled data on the habitat of these two populations, using published literature, inquiries to the public, and our own research from 1999 to 2016. During the period of our investigation, breeding at Etajima City was observed only once, in July 2000. In Takehara City, a distinct recruitment of juveniles has been observed since 2015. The compiled data on this horseshoe crab in Hiroshima Prefecture confirms that juveniles up to the tenth-instar stage inhabit the mud flats and that adults appear in the habitat only during the summer, as similarly observed for populations in other regions of Japan. The Takehara population was genetically characterized with the AT-rich region of mitochondrial DNA by only one haplotype, inferring that the Seto Inland Sea populations exhibit the founder effect.